Although about 20 million people in the United States suffer from migraine headaches, doctors say relatively few suffer from the ailment cited by Joe Gibbs when he retired yesterday as head coach of the Washington Redskins.

When announcing his retirement, Gibbs said that he had "migraine equivalent" and that, while he didn't have headaches, at the end of last season he didn't feel well and suffered from "other nervous reactions."

The 52-year-old Gibbs probably has what's known as "acephalic migraine," which indeed means "having the neurological syndrome of migraine without the headache," said Dr. William G. Speed III, associate professor at Johns Hopkins School of Medicine.

If so, the symptoms suffered by Gibbs could include slurred speech, weakness, numbness or tingling in the arms or legs, vertigo, seeing flashing lights or having other vision disturbances such as blind spots.

With migraines, after headaches, the most commonly experienced symptoms are those that affect vision, said Speed. "Sometimes you can see sparkles or flashing lights or have vision that is quite blurred or totally absent in certain areas."

Migraines, which may affect as much as 10 to 20 percent of the population, are a neurological disorder caused by genetic abnormalities involving both the nerve cells in the brain and the neurotransmitters, or the chemicals through which brain cells communicate.

Symptoms of the disorder occur "from time to time and often in unpredictable fashion," said Dr. Gregory K. Bergey, associate professor of neurology at the University of Maryland School of Medicine. Headaches are the most common and most incapacitating symptom of the disorder, he said.

Generally, migraine headaches can be treated by medication and by a restricted diet. Usually the patient would refrain from eating or drinking foods such as chocolate, aged cheese, coffee, tea and citrus fruits and juices, or foods prepared with monosodium glutamate, said Speed.

About 10 to 20 percent of those who suffer from migraine headaches also experience other symptoms, such as blurry vision, dizziness or numbness just before they get a headache, Bergey said. These symptoms are called the aura and "can be a warning because they precede the headaches."

An even smaller percentage of people who have the neurological disorder -- like Gibbs -- experience the blurriness of vision or other symptoms, but not the headaches.

"It is very uncommon," said Speed. "We see a large number of migraine patients throughout the year and two or three have the acephalic migraine."

However, the term " 'migraine equivalent' is a less well-defined complaint than simply migraine," said Bergey. "It can cover a broad range of symptoms and a lot of doctors use that term when they aren't sure what [the problem] is."